I thought he seemed too young initially, until someone mentioned how young he was in the book. I guess it's just about perceptions readers have.

Technically this isn't slang, it's a creole. In the books where it's written out you can clearly see that's it's mixed from various languages. There's no way to make that intelligible for solely English speakers without abandoning the way the book used it.

That "future speak slang" is a pretty big part of the books, it's the Belter patois and part of what makes them so alien. I like they way they handled it in the pilot. It should feel weird and confusing because it's supposed to be. It's the same way they used Gaidhlig in Outlander, and it worked well there.